Add indirection to avoid runtime dependency on MVC in templates

Velocity and Freemarker share some common properties so the base class for
configuring their properties makes some sense. Unfortunately the implementation
pulls in Spring MVC at runtime because of the signature of one method (that
would never be called). We can fix that in a number of ways, but the least
disruptive is probably to change the signature of that method and only refer
to the concrete template view resolver type if the method is called.

Fixes gh-1437

spring-boot-samples/pom.xml

@@ -48,6 +48,7 @@
 		<module>spring-boot-sample-tomcat-multi-connectors</module>
 		<module>spring-boot-sample-tomcat8-jsp</module>
 		<module>spring-boot-sample-traditional</module>
+		<module>spring-boot-sample-velocity</module>
 		<module>spring-boot-sample-web-freemarker</module>
 		<module>spring-boot-sample-web-groovy-templates</module>
 		<module>spring-boot-sample-web-method-security</module>